1. Field of the Invention
This invention relates generally to a spray apparatus and methods of applying coatings of polymers to application surfaces. More particularly, this invention relates to a method and apparatus for transforming a solid polymer into its molten state and transporting the molten polymer to a spray head for subsequent delivery, in combination with a heated pressurized gas stream, in the form of molten droplets. When the molten polymer droplets strike the application surface, they adhere and combine to form a solid coat of polymer upon cooling.
2. Description of Related Art
It has long been appreciated that thermoplastic polymer coatings offer advantages over solvent-based coatings for providing protection afforded the substrate and to the process (in elimination of solvent vapors to the environment). The coated substrate enjoys enhancement in adhesion, chemical resistance, flex strength/modules, impact resistance, and repairability, as well as providing a broader range of material properties in the polymer coating. The substrate to be coated can be any material relatively resistant to heat, including wood, metals, glass, fibrous glass reinforced synthetic resin, or even cardboard without damaging the material surface. Employing the invention apparatus for applying a polymer coating instead of methods employed in applying a solvent-based coating material offers the environmental advantages of (1) safe and easy transportation, storage, and handling of non-hazardous raw materials; (2) no volatile organic compound (VOC) emissions during application; (3) no hazardous waste generated; and (4) no toxic organic chemical solvents or thinners, as well as the advantages of (5) no messy overspray (with attendant product loss) and (6) no shelf or pot life restrictions.
The earliest thermoplastic polymer coatings were electrostatic powder coatings, which involved electrostatic attraction/attachment of the thermoplastic polymer in powder form onto the metallic surface and heating to temperatures causing the polymer to melt and flow to form a continuous film. While effective, this process suffers practical limitations. The coating cannot be applied in the field. The size of the item to be coated is limited to the size of the curing/melting oven. Further, the thickness of the coating is limited by the electrical insulation (reducing or eliminating the electrostatic attraction force) as the powder thickness increases.
Alternatively, it is known to coat substrate surfaces using flame (or thermal) coating technology. Known thermal spray processes are characterized by chemical combustion heating including powder flame spraying, wire/rod flame spraying, and detonation/explosive flame spraying, and by electrical heating processes including plasma flame spraying. Plasma flame spraying involves the use of an ionized gas consisting of free electrons, positive ions, atoms, and molecules as a means of heating a material, such as metal powder, to a molten state at a high temperature and depositing the metal as a coating on a substrate, such as a chrome plate on an automobile part.
There are a number of known devices for spraying powders of high temperature thermoplastics or other high temperature polymer coatings to a variety of surfaces such as U.S. Pat. No. 3,676,638, which discloses a nozzle whereby powder is fed into the plasma stream downstream from the arc. U.S. Pat. No. 2,774,625 teaches an apparatus which uses detonation waves in spraying powders. U.S. Pat. No. 3,111,267 teaches a thermal spray gun apparatus for applying heat fusible coatings on solid objects wherein powder material is fed directly through a heating zone in the spray in which it reaches a molten or, at least, a hot plastic condition and is then propelled at a relatively high velocity onto the object to be coated. U.S. Pat. No. 3,627,204 discloses a spray nozzle arrangement for plasma gun wherein powder material is fed into a spray nozzle downstream of an arc chamber. U.S. Pat. Nos. 4,004,735 and 4,231,518 teach apparatuses for a detonating application of coating with powdered material. U.S. Pat. No. 4,290,555 teaches a method for introducing powder into a gas stream to be provided to a burner. U.S. Pat. No. 4,370,538 teaches an apparatus for spraying heated powder and the like wherein the apparatus includes a combustion chamber which is cooled by air flowing through an annular passage. U.S. Pat. No. 4,688,722 discloses a nozzle assembly for a plasma spray gun. Also, U.S. Pat. No. 4,911,363 teaches a flame spray apparatus including a combustion head provided with radially spaced longitudinal channels extending inwardly from the periphery thereof along which water passes to cool the combustion head. Finally, U.S. Pat. No. 5,520,334 discloses an air and fuel mixing chamber for a tuneable, high-velocity, thermal spray gun.
While overcoming some of the limitations of electrostatic polymer coating processes, flame coating is inefficient in that it creates new concerns and presents practical limitations of its own. These concerns and limitations relate to the common requirements of all conventional thermal spray systems: first, an open flame (or the equivalent thereof) to melt the thermoplastic polymer; and, second, the necessity that the polymer fed to the spray system be in powder form. In addition to its high-cost, plastic powder is difficult to handle and is conducive to material loss.
It is manifest that any open flame is dangerous and presents serious hazards, both to the applicator and to anyone in his general vicinity. The industrial use of flame spray coating processes essentially amounts to placing flame throwers into the hands of workers in a manufacturing facility. Another impediment to the efficiency of such processes is that plastic is a good insulator. Melting the plastic presents a heat transfer problem. Transferring heat energy into plastic by way of conduction is inefficient. Even a very hot flame is a slow, inefficient solution to the basic heat transfer problem. As a result, most flame systems can spray only about ten (10) pounds of plastic per hour or less. To compound the inefficiency of this slow delivery, most flame spray systems result in only a part of the delivered material being applied to the target substrate material. The application process is dangerous, expensive, and slow.
The velocity of a low velocity flame spray chemical process produces a coating of low bond strength and uneven particle melt; wherein some of the thermoplastic particles are amorphous, and overheated particles are crystalline. The plastic particle""s exposure to heat energy is limited to its residence time in the flame. Each particle must reach its melt/sticky temperature during this residence time. Too short a residence time results in particles, that do not achieve this temperature, and thus do not stick to the target surface. The particles that do not stick to the surface fall off and become waste/scrap material. Too long a residence time results in particles that melt and then bum, or crystallize.
The problem of slow delivery has been addressed by one practitioner. Weidman, in U.S. Pat. Nos. 5,041,713 and 5,285,967, discloses high velocity thermal spray guns for spraying a melted powder of thermoplastic compounds onto a substrate to form a coating thereon. The latter patent, in particular, discloses a gun including a high velocity, oxygen fueled (HVOF) flame generator for providing an HVOF gas stream to a fluid cooled nozzle. The heat transfer problem is addressed by diverting a portion of the gas stream for preheating the powder, with the preheated powder being injected into the main gas stream at a downstream location within the nozzle. This method/apparatus approach to overcoming the heat transfer problem to produce a higher velocity spray still leaves concerns associated with the high temperature arc/flame exposure danger and the reliance on a thermoplastic polymer powder as the raw material.
The powder form of the thermoplastic polymer has continued as the material of choice for several reasons. Inasmuch as the powder is the only acceptable form of the material for the earlier electrostatic process for coating substrates, it was logical that the later developed high velocity delivery equipment be designed for the same form of material. Also, manufacturers and marketers of plastic flame coating equipment normally also manufacture and market thermoplastic polymer powder xe2x80x9cspecifically designedxe2x80x9d for their equipment. For example, one company""s flame coat powder xe2x80x9cNo. III,xe2x80x9d manufactured by Dupont and sold as Nucrelo(trademark), sells for $10.50 per pound. The same Nucrelo(trademark) material can be purchased in pellet form for $2.00 per pound. Therefore, the ability to use a larger particle size thermoplastic polymer material can provide a significant economic advantage.
The most common application of flame sprayed thermoplastic coatings is for the protection of metals against corrosion. A properly applied polymer coating is perhaps the most effective corrosion barrier available. For this performance, industries involved with corrosive materials, applications, and/or environments are willing to accept the various disadvantages discussed above. Nevertheless, there is a need for an improved method and/or apparatus for applying thermoplastic polymer compositions on substrate surfaces.
In particular, there is a need for the ability to apply a high volume of thermoplastic polymer coating in a short time. There is a need for a clean and efficient system that applies accurately with little or no waste from over spraying. There is a need for a system that is safe in an industrial environment, both from the perspective of safety for the user and for the facility. There is a need for the ability to apply a wide range of materials in various forms, such as pellets, regrind, recycled, or blended plastic materials, as well as powdered. A system which meets all these objectives is necessarily safer, environmentally friendlier, and more economical than currently available thermal spray systems.
The present invention is directed to an apparatus and method for spraying a molten thermoplastic polymer composition onto a substrate, preferably in the absence of a flame or a high-temperature arc. The thermal spray apparatus of the present invention includes a source of pressurized molten polymer material, a source of pressurized hot gas, and a spray head which is in fluid communication with the source of pressurized molten polymer material and the source of pressurized hot gas. The pressurized hot gas forms a flowstream as it exits the spray head and acts to atomize and transport the molten polymer material, in a molten state, to the substrate so that the substrate is coated. The molten polymer is atomized into relatively uniform particulates of molten plastic which aids in applying a uniform coating to the subject substrate.
The spray head has an input coating passage, a separate input air passage, and a nozzle assembly. The input coating passage is in fluid communication with the source of pressurized molten polymer coating material and the input air passage is in fluid communication with the source of pressurized hot gas. The nozzle assembly has a spray surface, a hot air receiving chamber, a plurality of air delivery conduits, and a coating material conduit. The hot air receiving chamber is in fluid communication with the input air passage. The air delivery conduits extend from the air receiving chamber to the spray surface of the nozzle assembly and define a plurality of air orifices. The air delivery conduits are in fluid communication with the hot air receiving chamber. Thus, when operational, hot pressurized gas exits the air orifices to form the flowstream which is at both a high temperature and high velocity.
The coating material conduit extends from the input coating passage to the spray surface of the nozzle assembly and defines a material orifice. The plurality of air orifices surround at least a portion of the material orifice so that, when operational, molten polymer exits the material orifice and subsequently interacts with the hot pressurized gas exiting the air orifices. The molten polymer is subsequently atomized by and transported to the substrate by the flowstream.